1. Field of the Invention
The present invention generally relates to an electronic component assembly (ECA) and to an associated method for making an electronic component assembly which can be used to eliminate capacitive coupling between multiple closely arranged printed wiring boards.
2. State of the Art
Printed wiring boards (PWBs) are usually arranged in a close or stacked manner in order to minimize their space requirements. These printed wiring boards are normally connected electrically to a chassis. Electrical circuits contained in the printed wiring boards have a common "circuit ground potential" while the chassis includes a separate "chassis ground potential". The chassis ground can provide a Faraday shield which protects the electrical circuits of the printed wiring boards from electromagnetic interference (EMI) of outside sources, such as radar energy.
The separate ground circuits are maintained to avoid electrical problems. For example, if the different ground elements of the circuit and the chassis are brought into contact, a ground loop can form between the printed wiring boards which raises the potential of the circuit ground above zero volts and renders the electrical circuits susceptible to noise. The proper operation of circuits which use discrete logic levels can be affected when noise distorts the circuit ground to a value greater than zero volts.
In an effort to reduce space and costs, it is desirable to place printed wiring boards physically closer to one another while still maintaining the separate grounds. To dissipate heat between plural, closely placed printed wiring boards, heat sinks are located between the printed wiring boards. This configuration can cause a capacitive coupling to occur between the printed wiring boards and the heat sink assembly. Because the speed of circuits has increased while the spacing between printed wiring boards has decreased, the capacitive coupling becomes more damaging to proper circuit operation as the influence of transient signals becomes more pronounced.
Capacitive coupling can be reduced in several different ways. For example, conductors can be placed between two adjacent printed wiring boards to short circuit the capacitive coupling. U.S. Pat. No. 5,251,099 (Goss et al.) discloses the use of connector pins which are attached to each circuit board and which connect with an adjoining circuit board. U.S. Pat. No. 5,353,191 (Volz et al.) discloses connectors mounted within the heat sink and provides a circuit interface between two printed circuit boards.
Capacitive coupling can also be reduced through the use of conductive adhesives. For example, U.S. Pat. No. 5,366,027 (Turek et al.) discloses a conductive tape placed between a heat sink and a dielectric in an attempt to reduce capacitive coupling.
Other attempts to reduce capacitive coupling include forming a separate ground layer on each printed wiring board. However, this technique requires further steps to make the circuit card assemblies thus increasing their cost and assembly time.
The foregoing methods of reducing the capacitive coupling are labor intensive and require a high degree of precision to avoid short circuits. Accordingly, an efficient, cost-effective technique to address the occurrence of capacitive coupling is needed.